Polyurethane binder for a magnetic recording medium

ABSTRACT

A polyurethane prepared from a functionalized polyol has superior hydrolytic stability and dispersive capacity which makes it useful as a binder for magnetic particles in a magnetic recording medium. The functionalized polyol may be one of the group consisting of a polyesterdiol, a polyetherdiol, a polycaprolactone diol and a polycarbonate diol, each of which incorporates a functional group having the formula: ##STR1## wherein M is a methylene group or nitrogen, R and R 1  are the same or different alkylene groups having from 1 to 7 carbon atoms, R 2  is an alkylene group having from 1 to 7 carbon atoms, and R 3  and R 4  are the same or different alkyl groups having from 1 to 7 carbon atoms, and X is an anion of a Br.o slashed.nsted acid 
     in an amount such that the polyurethane contains from about 8 to about 32 gram moles of the functional group per 1×10 6  grams of the polyurethane. The polyurethane may be made by heating a mixture of a Br.o slashed.nsted acid salt of a carboxylic acid terminated aminoester, a stoichiometric excess of a glycol, and a dicarboxylic acid to make a functionalized polyesterdiol and curing a mixture of said polyesterdiol and a diisocyanate.

This is a divisional of application Ser. No. 08/816,467 filed on Mar.13, 1997 now U.S. Pat. No. 5,747,630.

BACKGROUND OF THE INVENTION

This invention relates to magnetic recording media such as tapes anddiscs which are obtained by applying a magnetic coating on anon-magnetic support.

General purpose magnetic tapes and discs are produced by coating apolyethylene terephthalate film with a magnetic material prepared bydispersing ferromagnetic particles about 1 micron long in a resinousbinder. The resinous binder plays a very important role in providingexcellent dispersibility, filling efficiency, and orientation ofmagnetic particles as well as imparting excellent durability, abrasionresistance, heat resistance and smoothness to the magnetic coating andadhesion thereof to the support.

Examples of resinous binders conventionally used include vinylchloride/vinyl acetate copolymers, vinyl chloride/vinyl acetate/vinylalcohol copolymers, vinyl chloride/vinylidene chloride copolymers,polyurethane resins, polyester resins, acrylonitrile/butadienecopolymers, nitrocellulose, cellulose acetate butyrate, epoxy resins,and acrylic resins. Of these resins, conventional polyurethane resinshave excellent toughness and abrasion resistance compared to otherresins but often are inferior in properties such as blocking resistance,heat resistance, and running stability. For these reasons, a mixedsystem of polyurethane resins with nitrocellulose or vinylchloride/vinyl acetate copolymers is often used. The durability, heatresistance, and adhesive properties of polyurethanes may be improved bycuring with a polyisocyanate at from about room temperature to about 40°C. or higher over a long period of time after the application and dryingof the magnetic coating composition.

A magnetic recording layer having highly improved strength and otherproperties employs a binder resin comprising both a vinyl chloridecopolymer (e.g., a vinyl chloride/vinyl acetate/maleic anhydridecopolymer) and a polyurethane resin. Japanese Patent Publication No.59-8127 teaches the incorporation of a polar group into one or both ofthe constituent resins to enhance the dispersibility of ferromagneticpowders in such a binder.

The durability and abrasion resistance of conventional resinous bindersare still insufficient for use in video tapes, computer tapes, andfloppy discs, all of which are required to have high performance andhigh reliability. Demand for high density and high quality recordingmedia is increasing while smoothness is still desired. As the requiredsmoothness increases, the running durability has suffered and resinousbinders with higher durability must be developed. To do so, it has beenproposed to introduce multifunctional components into the polyurethaneswhich are reactive with the polyisocyanate; trimethylol propane anddiethanolamine exemplify such components. A serious drawback to thisapproach is that the dispersibility of the magnetic particles oftendecreases as the durability improves. The high recording density andhigh quality required for magnetic media have been supplied in recentyears by fine magnetic particles of metals and barium ferrite butdurability and dispersibility are still required of resinous binders forsuch particles.

A method for improving the dispersibility of the particles by theincorporation of metal sulfonate groups or metal salts of acidicphosphorus compounds is taught in Japanese Patent Publication Nos.57-3134 and 58-41564 and in Japanese Patent Publication (Kokai) No.61-48122. More recently, Yatsuka et al has taught in U.S. Pat. No.5,009,960 that the presence of such multifunctional components in theresin containing the metal sulfonate group or metal salt of an acidicphosphorus compound for the purpose of improving the durability of suchcoating still results in a lesser dispersibility. Yatsuka et al furthertaught that the incorporation of a bicyclic amide acetal into thepolyurethane resin will overcome the deficiencies of the prior art. Apreferred polyurethane contains, as a functional group, a metal salt ofan acidic phosphorus compound.

In U.S. Pat. No. 5,371,166, Farkas et al teach that the dispersibilityand hydrolytic stability of a polyurethane are improved by adding anaminodiol and a Br.o slashed.nsted acid to a chain extender and/orhydroxy-terminated polyol which are to be reacted with a diisocyanate tomake the polyurethane. The reaction product of the aminodiol and Br.oslashed.nsted acid thus does not become a part of the polyol but becomesinstead part of the hard segment of the polyurethane. Phosphoric acid,which gives an insoluble product, is not named as a Br.o slashed.nstedacid.

SUMMARY OF THE INVENTION

In view of this, the present inventor has studied extensively with theobject of improving the dispersibility and adsorptivity of magneticparticles in the resinous binder and improving the durability of themagnetic recording medium.

It is an object of this invention, therefore, to provide a durablemagnetic recording composition which has excellent dispersibility offerromagnetic particles which may be free of vinyl chloride polymers andcopolymers containing polar groups.

It is a related object of this invention to provide a magnetic recordingcomposition which has excellent hydrolytic stability.

It is a further object of this invention to provide a cationic magneticcoating composition having an attenuated viscosity to facilitate themanufacture of a magnetic recording tape.

It is related object of this invention to provide a magnetic recordingmedium having a very smooth layer of said recording composition on itssurface.

It is another related object of this invention to provide a magneticrecording medium having an improved signal-generating capacity.

It is another object of this invention to provide a method for makingsaid resinous binder.

These and other objects of the invention which will become apparent fromthe following description are achieved by a polyurethane whichincorporates a functionalized polyol selected from the group consistingof a polyesterdiol, a polycaprolactonediol, a polycarbonatediol, andmixtures thereof, and which incorporates a functional group having theformula: ##STR2## wherein M is a methylene group or nitrogen, R and R¹are the same or different alkylene groups having from 1 to 7 carbonatoms, R² is an alkylene group having from 1 to 7 carbon atoms, and R³and R⁴ are the same or different alkyl groups having from 1 to 7 carbonatoms, and X is an anion of a Br.o slashed.nsted acid

in an amount such that the polyurethane contains from about 8 to about32 gram moles of the functional group per 1×10⁶ grams of thepolyurethane.

DETAILED DESCRIPTION OF THE INVENTION

The magnetic coating composition of this invention may contain otherbinder resins such as those mentioned hereinabove. As used herein, theterm functionalized polyol means a hydroxyl terminated polyester,polyether, polycaprolactaone, and polycarbonate containing thefunctional group of Formula I and used as an intermediate in thepreparation of binder resin of this invention.

The functionalized polyesterdiols used in the preparation of apolyurethane are preferably linear and have a weight average molecularweight of from about 500 to about 1000, preferably from about 500 toabout 700. They may be produced by a conventional procedure in which oneor more dicarboxylic acids and one or more glycols are heated in thepresence of an acid catalyst until the acid number is reduced to about30 or less, preferably less than 1, more preferably less than 0.8. Theglycol to acid mole ratio is preferably greater than one so as to obtainlinear chains having a preponderance of terminal hydroxyl groups. Thehydroxyl number may be from about 50 to about 200 or more.Transesterification of a given dicarboxylic acid ester by reaction withone or more of such glycols is also suitable.

Preferably, the aminodiol is reacted with a stoichiometric excess of thedicarboxylic acid first to obtain a carboxylic acid terminated ester.After reaction of the carboxylic acid terminated aminoester with theBr.o slashed.nsted acid, a stoichiometric excess of one or more of theother ester forming glycols, with respect to the amount of unreacteddicarboxylic acid remaining, is added to the reaction mixture beforecontinuing the esterification as outlined above. A much shorter reactiontime is achieved in comparison with the procedure wherein the mixture ofglycols, aminodiol, and dicarboxylic acid is reacted.

The ester-forming glycols in this invention include the tertiaryaminodiols from which the cations of the functional groups of Formula Iare derived. Examples of aminodiols useful in forming the functionalmodifier include: N,N-dimethyl-N',N'-di-(2-hydroxypropyl)-1,3 propanediamine, 3-diethylamino-1,2-propanediol (DEAPD),3-dimethylamino-1,2-propanediol, 3-dipropylamino-1,2-propanediol;3-diisopropylamino-1,2-propanediol; 2-4-(dimethylamino)butyl!-1,3-propanediol; 3-dimethyl-1,6-hexanediol; and1-(dimethylamino)-2,2-(bis(hydroxymethyl)propane; as well as anycombinations thereof.

Other ester-forming glycols may be aliphatic, cycloaliphatic, aromaticor mixtures thereof. Examples of the glycol component include alkyleneglycols having from 2 to 10 carbon atoms as exemplified by ethyleneglycol, 1,2-propanediol, 1,4-butanediol, 2,6-hexamethylenediol, andmixtures thereof. ethylene glycol, propylene glycol, 1,3-propanediol,1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, neopentyl glycol,diethylene glycol, dipropylene glycol, 2-methyl-1,3-propane diol;2-butyl-2-ethyl-1,3-propane diol; 2,2,4-trimethyl-1,3-pentanediol;cyclohexanedimethanol, ethylene oxide and propylene oxide adducts ofbisphenol A, ethylene oxide and propylene oxide adducts of hydrogenatedbisphenol A, polyethylene glycol, polypropylene glycol,polytetramethylene glycol, and the like.

The dicarboxylic acid component of the polyesterdiol may be aliphatic,cycloaliphatic, aromatic or mixtures thereof. Aromatic dicarboxylicacids are exemplified by terephthalic, isophthalic, orthophthalic or itsanhydride, 1,5-naphthalic acid, oxycarboxylic acids such as p-oxybenzoicacid, and p-(hydroxyethoxy)benzoic acid. Suitable aliphatic dicarboxylicacids include succinic, adipic, azelaic, sebacic, and dodecanedioc acid,and alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid,hydrogenated 2,6-naphthalenedicarboxylic acid and the like. An anhydridemay be used in place of or with the acid.

The polycaprolactonediols used as intermediates in the preparation of afunctionalized polyurethane of this invention, are made by the reactionof a caprolactone of one of the above-described functionalizedpolyesterdiols and a bifunctional compound having two reactive sitescapable of opening the lactone ring, also exemplify functionalizedpolyols suitable for this invention. The bifunctional compounds may berepresented by the formula HY--R'--YH wherein R' is an aliphatic,cycloaliphatic, aromatic, or heterocyclic radical and Y is O, NH, or NR"where R" is alkyl, aryl, aralkyl, or cycloalkyl. Diols, diamines, andaminoalcohols are among the preferred compounds for this purpose. Thediols suitable for opening the lactone ring include the aminodiolsexemplified above and alkylene glycols having from 2 to 10 carbon atomsas exemplified by ethylene glycol, 1,2-propane-diol, 1,4-butanediol,2,6-hexamethylenediol, and mixtures thereof. The lactones arepolymerized readily by heating them with the bifunctional compounds to atemperature of from about 100° to about 200° C. Suitably, thepolycaprolactone diols have a weight average molecular weight of fromabout 500 to about 1000, preferably from about 500 to about 700.

Functionalized polycarbonatediols used as intermediates in thisinvention are made from diols such as those set forth hereinabove,including the aminodiols, by transesterification with low molecularweight carbonates such as diethyl or diphenyl carbonate or mixturesthereof, followed by reaction with a Br.o slashed.nsted acid. Generally,the polycarbonatediols have a weight average molecular weight of fromabout 500 to about 1000, preferably from about 500 to about 700.

The functionalized polyether diols are derived from a diol having atotal of from 2 to 15 carbon atoms, including an aminodiol as describedabove, by reaction with an ether comprising an alkylene oxide havingfrom 2 to 6 carbon atoms, typically ethylene oxide or propylene oxide,or mixtures thereof. For example, hydroxyl functional polyethers can beproduced by first reacting propylene glycol with propylene oxidefollowed by subsequent reaction with ethylene oxide. The aminogroup-containing polyol is then reacted with a Br.o slashed.nsted acid.Primary hydroxyl groups resulting from ethylene oxide are more reactivethan secondary hydroxyl groups and thus are preferred. The variouspolyether intermediates generally have an average molecular weight, asdetermined by assay of the terminal functional groups, of from about 500to about 1000. A preferred average molecular weight is in the range ofabout 500 to about 700.

Phosphoric acid is the preferred Br.o slashed.nsted acid for thepurposes of this invention. Other useful Br.o slashed.nsted acidsinclude monoalkyl-diacid phosphates, phosphonic acids, phosphinic acids,and sulfonic acids. Examples of phosphonic acids include but are notlimited to: methylphosphonic acid; ethylphosphonic acid;propylphosphonic acid; butylphosphonic acid; t-butylphosphonic acid;methylenediphosphonic acid; 2-chloroethylphosphonic acid; phenylphosphonic acid; phosphonoacetic acid; and phosphonopropionic acid aswell as any combination thereof. Illustrative examples of phosphinicacids include: phenylphosphinic acid; diphenylphosphinic acid;dimethylphosphinic acid; and bis(4-methoxyphenyl) phosphinic acid.Possible sulfonic acids to be used include: methanesulfonic acid;ethanesulfonic acid; propanesulfonic acid; sulfoacetic acid;sulfosuccinic acid; benzenesulfonic acid; 4-ethylbenzenesulfonic acid;4-hydroxybenzenesulfonic acid; 4-chlorobenzenesulfonic acid;p-toluenesulfonic acid; 4-sulfophthalic acid; 1-naphthalenesulfonicacid; 2-naphthalenesulfonic acid; 3-sulfobenzoic acid; 4-sulfobenzoicacid; and 5-sulfoisophthalic acid as well as any combinations thereof.Butyl diacid phosphate is representative of the useful monoalkylphosphates.

The functionalized polyurethane used as binder resins according to thisinvention has a weight average molecular weight of from 20,000 to75,000, preferably from 25,000 to 60,000. It is obtained by the reactionof one or more of the functionalized polyols described above, a chainextender having a molecular weight of less than 1000, and a diisocyanateat a temperature of from about 100° C. to about 270° C. until curing issubstantially complete. A temperature of about 120° C. for a time ofabout 90 is suitable. The ratio of isocyanate group to hydroxyl group isin the range of 1:2 to 1:1. Said ratio is a factor in determining themolecular weight of the resin. When the isocyanate content is too large,the resulting polyurethane is isocyanate-terminated and has a poorstorage life. When the hydroxyl content is too large, the molecularweight decreases. A preferred range is from 1:1 to 1:1.2. The reactionmay be carried out in either the molten state or in solution. The polyaddition reaction for producing the polyurethane of this invention maybe of the one-shot procedure wherein all of the components are reactedat one time, and the prepolymer method wherein a long chain diol isfirst reacted with excess isocyanate and the resultingisocyanate-terminated prepolymer is polymerized using a chain extender.The block polymer method is a variation on the prepolymer method whereinanother long chain hydroxyl group containing compound is reacted withthe isocyanate-terminated prepolymer. Stannous octylate, stannousoxalate, dibutyltin dilaurate, triethylamine, and the like may be usedas a catalyst. To prevent gelation of the polyurethane,p-toluenesulfonic acid, or an inorganic acid may be used. Ultravioletlight absorbers, hydrolysis inhibitors, antioxidants, and other usefuladditives may be added before, during, or after the production of thepolyurethane. The polyurethane may be a segmented block or randomcopolymer comprising a hard segment and a soft segment. The functionalgroup having FORMULA I may be incorporated into the soft segment byincluding one of the aminodiols named above in the mixture of glycolsused in the reaction with a dicarboxylic acid to make the polyesterdiol,thence the polycaprolactonediol, or the reaction with a dialkylcarbonate to make a polycarbonatediol and then reacting the resultingamino group-containing polyol with one of the Br.o slashed.nsted acidsmentioned above. It is preferred to use the Br.o slashed.nsted acid saltof an carboxylic acid terminated aminoester, as described above, in thepreparation of the polyurethane.

The chain extender has the effect of regulating the urethane groupcontent of the polyurethane resin to impart toughness to the resin.Examples of the chain extender include bifunctional to tetrafunctionalpolyols having a molecular weight of about 500 or less, particularlystraight chain glycols such as ethylene glycol; 1,3-propylene glycol;2-methyl-1,3-propane diol; 2-butyl-2-ethyl-1,3-propane diol;1,4-tetramethylene glycol; 1,6-hexanediol, cyclohexanedimethanol,xylylene glycol, diethylene glycol; triethylene glycol; and an ethyleneoxide adduct of bisphenol A; branched chain glycols such as propyleneglycol; neopentyl glycol; 1,2-butanediol; 1,3-butanediol;2,2,4-trimethyl-1,3-pentanediol; and a propylene oxide adduct ofbisphenol A; water; aminoalcohols such as monoethanolamine andN-methylethanolamine; diamines such as ethylene diamine, hexamethylenediamine, and isophorone diamine are suitable in some instances, but toavoid crosslinking, secondary diamines such as N, N'-dialkylphenylenediamine, and p, p'-di(alkylamino)diphenylmethane, sold underthe trademark Unilink by UOP, and piperazine and the like are preferred.The amount of chain extender is determined in part by the size andnature of the chain extender and in part by the desired properties.Trifunctional chain extenders such as trimethylolpropane,diethanolamine, triethanolamine, and glycerin may also be used with careas to their effect on the performance characteristics of thepolyurethane.

The diisocyanate is exemplified by 2,4-tolylene diisocyanate;2,6-tolylenediisocyanate; p-phenylenediisocyanate;diphenylmethanediisocyanate or MDI; m-phenylene diisocyanate;hexamethylenediisocyanate; tetramethylenediisoacyanate;3,3'-dimethoxy-4,4'-biphenylenediisocyanate; 2,4-naphthalenediisocyanate;3,3'-dimethyl-4,4'-biphenylenediisocyanate;4,4'diphenylenediisocyanate; 4,4'-diisocyanate-diphenyl ether,1,5-naphthalenediisocyanate; p-xylylenediisocyanate; m-xylylenediisocyanate; 1,3-diisocyanatomethylcyclohexane;1,4-diisocyanatomethylcyclohexane; 4,4'-diisocyanatodicyclohexane;4,4'-diisocyanatodicyclohexylmethane; isophorone diisocyanate, and thelike. Triisocyanates, such as 2,4-tolylenediisocyanate trimer andhexamethylenediisocyanate trimer and the like, are also used with careas to their effect on the performance characteristics of thepolyurethane.

The ferromagnetic particles used in the coating composition of thepresent invention include magnetic metal powders, γ-Fe₂ O₃, γ-Fe₂ O₃/Fe₃ O₄ mixed crystal, CrO₂, cobalt ferrite, barium ferrite, cobaltcontaining iron oxide, and ferromagnetic alloy powders such as Fe-Co,Fe-Co-Ni, pure iron, surface-treated iron (e.g., Fe-Co-Co-Ni, Fe-Co-Ni,Fe-Co-B, Fe-Co-Cr-B, Mn-Bi, Mn-Al, Fe-Co-V), iron nitride and othersimilar magnetic particles.

A solvent is generally used in the production of a magnetic coatingcomposition of this invention. Ketones such as methyl ethyl ketone,methyl isobutyl ketone, and cyclohexanone; esters such as methylacetate, ethyl acetate, and ethyl butyrate; and xylene; and mixtures oftwo or more of the preceding solvents are examples of those that areuseful. Further, exemplary solvents usable for the preparation of themagnetic coating formulation include the acetate esters of glycolethers, e.g., ethylene glycol monoethyl ether acetate and propyleneglycol monomethyl ether acetate; aromatic hydrocarbons, e.g., benzene,toluene and xylene; nitropropane; tetrahydrofuran; dimethylacetamide;and dimethylformamide and mixtures thereof.

A variety of materials conventionally known as additives for magneticcoating formulations can be suitably used, such as plasticizers,lubricants, abrasives, dispersants, antistatic agents and fillers.Dibutyl phthalate and triphenyl phosphate exemplify the plasticizers.Dioctyl-sulfosodium succinate, t-butylphenol polyethylene ether, sodiumethylnaphthalenesulfonate, dilauryl sulfate, zinc stearate, soybean oillecithin, myristic acid, butyl myristate and silicone oil exemplify thelubricants and antistatic agents.

The functionalized polyurethane contains a sufficient amount of thetertiary aminodiol residue which furnishes the cation of the functionalgroup of FORMULA I to yield a binder having a Br.o slashed.nsted acidsalt concentration of from about 8 to about 32, preferably from about 10to about 20, gram equivalents per 1×10⁶ grams of polyurethane. The veryhigh solution viscosity of functionalized polyurethanes of similarmolecular weight in the prior art, caused by high functional grouplevels, makes them very difficult to handle. The functionalizedpolyurethane of this invention provides dispersions of magneticparticles offering good dispersibility, adsorptivity, and low viscosity.It also provides good hydrolytic stability. Accordingly, this novelpolyurethane can be used advantageously as a dispersing binder foradvanced recording media using high surface area pigments and magneticparticles.

The magnetic recording medium of this invention comprises a non-magneticsupport and a magnetic coating containing magnetic particles dispersedin the novel highly dispersive binder of this invention. Material forthe support includes polyesters, polypropylene, cellulose triacetate,polycarbonate, poly(vinylchloride), and aluminum. Examples of suitablefilms of polyethylene terephthalate are described in U.S. Pat. Nos.4,454,312; 4,595,715; and 4,693,932.

Among the devices for dispersing the components of the magnetic coatingcomposition there may be mentioned a twin roll mill, a ball mill, apebble mill, vertical and horizontal sandmills and a high speed stonemill.

Methods for coating are exemplified by the knife coating, wire barcoating, doctor blade coating, reverse roll coating, and calendarcoating methods. After the magnetic coating has been coated onto thenon-magnetic support surface, the coated film is generally subjectedbefore drying to an orientating treatment in a magnetic field and to asmoothing treatment. The magnetic coating layer is from about 1 micronto about 12 microns thick and provides a magnetic field of from about600 to about 5000 gauss.

The binder resin of the present invention becomes a uniform resinsuperior in dispersibility of the magnetic particles by virtue of theincorporation of the amount prescribed above of the Br.o slashed.nstedacid salt of FORMULA I in the soft segment. As a result, the magneticrecording medium of this invention is superior in the fillingcharacteristics and orientation of the magnetic particles and thesmoothness of the magnetic layer of the recording medium.

The binder resins, magnetic coating composition, and magnetic recordingmedium of this invention are illustrated specifically in the followingexamples wherein all parts are parts by weight unless otherwiseindicated.

EXAMPLE 1

A mixture of 169 parts of deionized water, 10.0 parts of IRGANOX 1076antioxidant, 27.6 parts (0.1264 mole) ofN,N-dimethyl-N',N'-di-(2-hydroxypropyl)-1,3 propane diamine, and 1169.2parts (8.0 moles) of adipic acid was heated at a temperature of 170-180°C. in a reaction vessel equipped with a distillation column to drive offwater as it was formed by the condensation reaction. Nine parts (0.078mole) of concentrated phosphoric acid were added slowly to thecarboxylic acid terminated ester and the mixture was stirred for 0.5hour. Cyclohexanedimethanol, 1873.29 parts (13.0 moles) was added andthe mixture was heated at 170-180° C. under reduced pressure for twohours. FASCAT 2001 catalyst, 0.75 part, was added and the reaction wascontinued at a reduced pressure of about 5-10 mm Hg for two hours toattain an acid number of less than 13 and an OH number of about 200 toform a functionalized polyesterdiol which is useful as an intermediatefor this invention.

EXAMPLE 2

The functionalized polyesterdiol of Example 1 (49.30 parts) was mixedwith 40.34 parts of an adipic acid/1,4-butanediol polyesterdiol havingan OH number of 198 and 10.36 parts of 2-butyl-2-ethyl-1,3-propanediol(BEPD). This mixture was then heated with 55.93 parts ofdiphenylmethanediisocyanate (MDI) to yield a functionalized polyurethaneof this invention containing 10 gram moles of the phosphoric acid saltof the aminodiol residue in the soft segment per 1×10⁶ grams of thepolyurethane.

EXAMPLE 3

A first binder solution containing 289.85 parts of the vinyl chloridecopolymer containing sodium sulfonate groups (sold under the trademarkUC 569 by Union Carbide) in a solvent consisting of 547.5 parts each ofmethylethyl ketone (MEK), cyclohexanone (CHO), and toluene was preparedfor use in the millbase having the formulation shown in Table 1.

                  TABLE 1    ______________________________________    COMPONENT        WEIGHT    ______________________________________    Iron oxide LX-313                     3000.0    Carbon black     100.0    Aluminum oxide   60.0    MEK              876.33    CHO              876.33    Toluene          876.33    Myristic acid    45.0    Binder Solution  1932.35    ______________________________________     The millbase density was 1.32 grams/cc.

A letdown solution of 289.85 parts of a polyurethane of this inventionmade according to the general procedure of Example 2 was made bydissolving it in 1642.5 parts of a solvent made up of equal parts ofMEK, CHO, and toluene. To this there were added 45.0 parts of butylmyristate and 620.49 parts each of said MEK, CHO, and toluene mixture.

Seven hundred and seventy (770) parts of the millbase and 381 parts ofthe letdown solution are mixed for five minutes in an air driven mixerwith a propeller and this is then milled in a 250 ml BYK SL horizontalsand mill for 20 minutes at a shaft speed of 3000 rpm to give a magneticrecording composition of this invention.

The subject matter claimed is:
 1. A magnetic recording medium comprisinga non-magnetic support and a magnetic coating thereon, said coatingcomprising a binder resin and magnetic particles dispersed therein, saidbinder resin comprising a polyurethane incorporating at least onefunctionalized polyol selected from the group consisting of apolyesterdiol, a polyetherdiol, a polycaprolactone diol, and apolycarbonate diol, which polyol incorporates a functional group havingthe formula: ##STR3## wherein M is a methylene group or nitrogen, R andR¹ are the same or different alkylene groups having from 1 to 7 carbonatoms, R² is an alkylene group having from 1 to 7 carbon atoms, and R³and R⁴ are the same or different alkyl groups having from 1 to 7 carbonatoms, and X is an anion of a Br.o slashed.nsted acidin an amount suchthat the polyurethane contains from about 8 to about 32 gram moles ofthe functional group per 1×10⁶ grams of the polyurethane.
 2. Themagnetic recording medium of claim 1 wherein R and R¹ are propylenegroups.
 3. The magnetic recording medium of claim 2 wherein R² is apropylene group.
 4. The magnetic recording medium of claim 3 wherein R³and R⁴ are alkyl groups.
 5. The magnetic recording medium of claim 4wherein the amount of the functional group is from about 10 to about 20gram moles per 1×10⁶ grams of polyurethane.
 6. The magnetic recordingmedium of claim 4 wherein the Br.o slashed.nsted acid is phosphoricacid.
 7. The magnetic recording medium of claim 3 wherein the amount ofthe functional group is from about 10 to about 20 gram moles per 1×10⁶grams of polyurethane.
 8. The magnetic recording medium of claim 3wherein the Br.o slashed.nsted acid is phosphoric acid.
 9. The magneticrecording medium of claim 8 wherein the amount of the functional groupis from about 10 to about 20 gram moles per 1×10⁶ grams of polyurethane.